The gains from RISs highly rely on the knowledge of the channel state information (CSI). However, the accurate acquirement of the CSI in RIS-aided systems is a challenging task, since RISs are typically passive without RF chains.
To cope with this challenge, researchers have turned their attention to designing the phase shifts of RISs based on statistical CSI [1]}, [2]}, [3]}. Thanks to the slowly changing nature of the statistical CSI, we only need to configure the phase shifts of the RIS once over a large time-scale. Accordingly, this greatly reduce the computational complexity, and reduce the overhead of channel estimation and feedback.
